Rare-earth magnets containing rare-earth elements such as lanthanoide are called permanent magnets as well, and are used for motors making up a hard disk and a MRI as well as for driving motors for hybrid vehicles, electric vehicles and the like.
Indexes for magnet performance of such rare-earth magnets include remanence (residual flux density) and a coercive force. Meanwhile, as the amount of heat generated at a motor increases because of the trend to more compact motors and higher current density, rare-earth magnets included in the motors also are required to have improved heat resistance, and one of important research challenges in the relating technical field is how to keep magnetic characteristics of a magnet at high temperatures.
The following briefly describes one example of the method for manufacturing a rare-earth magnet. For instance, in a typically available method, Nd—Fe—B molten metal is solidified rapidly to be fine powder, while pressing-forming the fine powder to be a compact. Hot deformation processing is then performed to this compact to give magnetic anisotropy thereto to prepare a rare-earth magnet (orientational magnet).
The hot deformation processing is performed by placing a compact between upper and lower punches, for example, followed by pressing with the upper and lower punches for a short time such as about 1 second or less while heating, so that processing is performed with the ratio of processing of at least 50% or more. Such hot deformation processing can give magnetic anisotropy to the compact, but has a problem that, during the course of the compact being crushed while being plastic-deformed by the pressure from the upper and lower punches in the hot deformation processing, the plastic deformed compact tends to generate cracks (including micro-cracks) at the side faces.
This results from excessive deformation of a part of the compact that comes into contact with the upper and lower punches, and accordingly excessive swelling occurs at the central part at the side faces, i.e., the deformation shaped like a barrel as one reason. Such cracks cause the processing deformation that is formed to improve the degree of orientation to be open at the positions of the cracks, thus failing to direct the deformation energy to the crystalline orientation sufficiently. As a result, an orientational magnet obtained cannot have high degree of orientation (such high degree of orientation means high degree of magnetization).
Due to such cracks generated at the periphery, an orientational magnet that is shaped by hot deformation processing is cut out at a central part of predetermined dimensions that is free from cracks for a product, which means low material yield unfortunately.
Then as a conventional technique to solve such a problem of cracks generated during hot deformation processing, Patent Literature 1 discloses a manufacturing method. This manufacturing method is to enclose the compact as a whole into a metal capsule, followed by hot deformation processing while pressing this metal capsule with upper and lower punches. They say that this manufacturing method can improve magnetic anisotropy of the rare-earth magnet. Such a technique of performing hot deformation processing while enclosing a compact into a metal capsule is disclosed in Patent Literatures 2 to 5 as well.
When the compact as a whole is completely enclosed with a metal capsule, however, lateral plastic deformation of the compact due to pressure applied vertically is extremely constrained, and so no cracks are generated at the side faces of the compact after the plastic deformation, but this leads to another problem that it is difficult to achieve sufficient plastic deformation, resulting in the difficulty in obtaining high degree of orientation. For instance, in the case of a cylindrical-columnar compact having the upper face, the lower face and the circumferential side face, this is caused by, when the side-face area of the metal capsule corresponding to the side face of the compact is plastic-deformed laterally, the upper-face area and the lower-face area that are integrated with the side-face area, corresponding to the upper face and the lower face of the compact, constraining the stretching of the side-face area.
None of the aforementioned Patent Literatures mention the rate of strain, and assume that hot deformation processing is performed with the rate of strain of 0.1/sec or more and the ratio of processing of 50% or more (e.g., 70% or more), cracks cannot be prevented completely. This is because, when processing is performed with the rate of strain of 0.1/sec or more while covering the entire face with a steel material of a predetermined thickness or more by welding, impact receiving the magnet structure is too strong, or when the compact is cooled, the compact subjected to hot deformation processing is strongly constrained by the metal capsule as described above due to a difference in heat expansion. To solve this problem, Patent Literature 6 discloses the technique of making a metal capsule thinner by forging through multiple steps, and the embodiment disclosed uses an iron plate of 7 mm or more in thickness. This cannot prevent cracks completely, and additionally the shape of the magnet after forging cannot be said a near net shape, which requires finish processing at the entire face, thus worsening a problem, such as a decrease in material yield and an increase in processing cost.
When the thickness of a metal capsule covering the entire face of a compact completely is made thinner as disclosed in Patent Literature 1, for example, such a metal capsule will be damaged at the rate of strain of 1/sec or more, which causes discontinuous unevenness at the compact and so causes a disturbance of orientation. In this way, such a method cannot be said a preferable method.